In recent years, development of hybrid vehicles and electric vehicles has rapidly been advancing. With this development, instead of the conventional mechanical braking, electrical braking has rapidly been developed.
When a battery is used as a power source for electrically controlling a vehicle, an auxiliary power source, especially one using electric double layer capacitors, is usually incorporated other than the battery against an accidents in which the battery cannot supply power.
In an auxiliary power source using electric double layer capacitors, a plurality of capacitors are connected in series because the withstand voltage of the capacitor is relatively low. When the capacitors connected in series are charged, the voltage of each capacitor is controlled separately so that dispersion in the characteristics of the capacitors does not concentrate voltage only on a specific capacitor and the voltage of each capacitor does not exceed the withstand voltage.
For example, in Japanese Patent Unexamined Publication No. 2001-292507, the voltages of a plurality of capacitors connected in series are controlled separately. However, separately controlling the voltages of respective capacitors poses a problem of a larger scale of a circuit.